1. Field of the Invention
The present invention relates generally to cellular mobile communications systems, and more specifically to a base station of a code division multiple access (CDMA) cellular mobile communication network when performing soft handoff for a mobile station which is encountering sudden variations of field strength profiles.
2. Description of the Related Art
In conventional CDMA cellular networks as standardised by the IS-95 standard and adopted by the TIA (Telecommunications Industry Association) and the EIA (Electronic Industries Association), it is the usual practice to share the same frequency by mobile stations and base stations. Two predetermined high and low thresholds respectively called xe2x80x9caddxe2x80x9d and xe2x80x9cdropxe2x80x9d thresholds are set in mobile stations. When a mobile station is moving around while communicating with a base station A and entering the coverage of an adjacent base station B, the mobile station is constantly monitoring pilot signals from surrounding base stations and transmits a report message called xe2x80x9cPilot Strength Measurement Report Messagexe2x80x9d to the base station A at the instant the strength of a surrounding base station""s pilot signal exceeds the xe2x80x9caddxe2x80x9d threshold.
This report message contains identifications of all base stations whose pilot signal strengths at the mobile station are higher than the xe2x80x9caddxe2x80x9d threshold and the identification of the currently communicating base station. The report message is relayed through the base station A to the base station controller, which responds with a message called xe2x80x9cTraffic Channel Startup (TCS) Messagexe2x80x9d transmitted to the base station B which was listed in the PSM report message (see FIG. 1). When an acknowledgment signal called xe2x80x9cTraffic Channel Startup Acknowledgment (TCSA) Messagexe2x80x9d is received from the base station B, the base station controller directs the base station A to send a Handoff Direction Message to the mobile station, as indicated by steps 1 to 4 in FIG. 1. However, if the mobile station encounters sudden variations in the field strength profiles of the first and second base stations, the Handoff Direction Message from the first base station may not reach the mobile station and its call is dropped.
For example, if a building is located at or near the boundary between the base stations A and B, the transmission from the base station A creates high and low field strength areas on opposite sides of the building and the transmission from the base station B creates high and low field strength profiles which are in reverse to the profiles of the base station A. If the mobile station is changing its location from one side of the building to the other side while communicating with the base station A, the mobile station will encounter a sudden decrease in the field strength of the pilot signal from base station A and a sudden increase in the field strength of the pilot signal from base station B. If the pilot signal from base station B exceeds the xe2x80x9caddxe2x80x9d threshold as shown in FIG. 2, the mobile station reports the identifications of the base stations A and B to the base station controller via the base station A. After exchanging a Traffic Channel Startup Message and a Traffic Channel Startup Acknowledgment Message with the base station B, the base station controller sends a Handoff Direction Message to the mobile station via the base station A. If the mobile station has moved past the building, the Handoff Direction Message from the base station A will be hindered by the building and not detected by the mobile station.
Therefore, a need exists to provide a handoff operation that can be successfully established even if a mobile station encounters sudden variations in field strength profiles.
Japanese Laid-Open Patent Specification Hei-9-191276 discloses a CDMA base station transceiver. In the prior art CDMA transceiver, the base station has a power control channel for controlling the transmission power of a mobile station so that the transmission power of its control data is higher than that of its speech signal. Since the Handoff Direction Message is not transmitted on this control channel, the prior art technique is not useful for handoff operation.
It is therefore an object of the present invention to provide a CDMA cell-site base station and a method for performing a soft handoff operation which ensures against a failure which would otherwise occur when a mobile station encounters sudden field strength variations at the cell boundary of adjacent base stations.
According to a first aspect of the present invent ion, there is provided a base station for a CDMA cellular mobile communication network. The base station comprises an encoder for encoding signals from a base station controller, the signals including a first or a second control message multiplexed with a user signal on a traffic channel assigned to a mobile station. The first control message indicates that a soft handoff operation is initiated and the second control message indicates that the soft handoff operation is being terminated. A variable gain amplifier is coupled to the encoder and a transmitter is coupled to the output of the amplifier for transmission of a spread spectrum signal to the mobile station. A control message detector is provided for detecting when the first control message is being encoded by the encoder and controlling the amplifier so that the encoded first control message is amplified with a gain higher than a gain with which the encoded user signal of the traffic channel is amplified. As a result, the first control message is transmitted at a power level sufficient to reach the mobile station, which would otherwise fail to receive this message when it encounters a sharp decrease in field intensity.
According to a second aspect, the present invention provides a method of performing a soft handoff operation in a CDMA cellular mobile communication network, the method comprising the steps of monitoring, at a mobile station communicating with a first base station, first and second pilot signals respectively sent from the first base station and a second base station, transmitting from the mobile station a reporting message indicating an identification of the second base station if the strength of the second pilot signal is higher than a predetermined level, receiving the reporting message at a base station controller via the first base station and exchanging information between the base station controller and the second base station, multiplexing a handoff direction message with a user signal on a traffic channel and transmitting the traffic channel from the base station controller to the first base station, the handoff direction message indicating that a soft handoff is initiated, and retransmitting the handoff direction message and the user signal from the first base station to the mobile station such that the retransmitted message has a power level higher than a power level at which the user signal is transmitted. Responsive to the handoff direction message, a handoff completion message is transmitted from the mobile station to the base station controller via one or both of the first and second base stations.